Fail-safe reflected light-responsive control circuit



y 11, 1967 F. w. WILLARDSON EJAL 3,330,959

FAIL-SAFE REFLECTED LIGHT-RESPONSIVE CONTROL CIRCUIT Filed May 27, 196465. ZQQ O HUN INVENTOR FRED W WILLARD o/v at" BY .EDWARDZ ZMUDA ATTORNEYUnited States Patent ABSTRACT OF THE DISCLOSURE Control circuitryincludes a device responsive to the magnitude of reflected light tocause development of a control signal. Also, normal operation of thecircuitry is dependent upon normal functioning of the components andoperation if the circuitry fails upon failure of a component.

This invention relates to a control circuit and more particularly to afail-safe control circuit employing a device responsive to variations inillumination level.

Many types of apparatus and machinery are available for performingrepetitive operations, and such equipment usually includes some type ofcontrol circuitry for the purpose of interrupting and stopping theequipment in the event that the operation becomes nonrepetitive ornonuniform. Moreover, much of the control circuitry in use todayincludes some type of photoconductive device responsive to variations inthe amount of illumination focused thereon to provide a signal which canbe used to stop or control the equipment.

However, it has been found that a great many of the available controlcircuits employing photoconductive devices are inadequately safe-guardedand for this reason leave much to be desired. For instance, many suchcircuits have little or no provisions therein which insure thediscontinuance of the operation of the apparatus should one of thecomponents in the circuitry fail to function. In other words, theapparatus which is supposedly controlled by the circuitry continues tooperate eventhough the circuitry designed to protect and interrupt theoperation is no longer functioning.

Therefore, it is an object of this invention to enhance thedependability of a control circuit having a photoconductive devicetherein.

Another object of the invention is to insure the functional operation ofa control circuit during the operational period of the apparatuscontrolled.

A further object of the invention is to actively engage all of thecomponents of a control circuit during the operational period of theapparatus being controlled and to discontinue the operation of theapparatus upon failure of one of the components in the controlcircuitry.

These and other objects are achieved in one aspect of the invention by acontrol circuit wherein virtually any failure in circuit components willactivate the control features and interrupt the operation of theapparatus. Also, the control features of the circuit are responsive tothe signal provided by a photoconductive device activated by variationsin the level of illumination applied thereto.

For a better understanding of the present invention, together with otherand further objects, advantages, and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe accompanying drawing in which is shown a schematic illustration ofone embodiment of the invention.

Generally, the circuitry includes a device responsive to the level ofillumination reflected thereon from a surface whereon a light source isdirected. This device is so designed that a shift in the level ofreflected illumination causes a signal which activates a currentresponsive de- 3,335,959 ,Patented July 11, 1967 vice and eventually thecontrol circuitry necessary to discontinue the operation of thecontrolled apparatus. Also, the control circuitry is designed such thatthe failure of a component therein activates the circuitry andinterrupts the operation of the apparatus.

Referring to the drawing, there is illustrated a control circuit 3 whichincludes a reflective lighting system 5 and a light-tight container 7wherein is contained a transformer 9, a voltage divider 11, a pair ofvoltage responsive devices 13 and 15, and a pair of current responsivedevices 17 and 19.

The lighting system 5 is external to the light-tight container 7 as willbe explained hereinafter and the container 7 includes therein atransformer 9 having a primary winding 21 for'connection to a source(not shown) of alternating potential and a plurality of secondarywindings including a first winding 23, a second winding 25, and a thirdwinding 27. This third winding 27 has a center tap 29 connected directlyto circuit ground as is common in the art.

The first winding 23 provides a source of low potential such as, forinstance, 6.3 volts wherefrom the illumination source 31 of the lightingsystem 5 is energized. This illumination source 31 bridges the winding23 and may be any one of a number of light sources including an ordinaryincandescent lamp.

The second winding 25 is bridged by a regulating means 32 which includesa series connected resistor 33 and a pair of neon tubes 35 and 37 Whichcooperate to provide a substantially uniform and regulated voltageacross the tubes 35 and 37 even though the potential of the source (notshown) is subject to variations. The tubes 35 and 37 are shunted by avoltage divider 11 which includes a series connected light responsivedevice 39 and a resistor 41 having a tap 43 intermediate theretodirectly connected to circuit ground.

Connected across the voltage divider 11 by way of a pair of currentlimiting resistors 45 and 47 is a pair of voltage responsive devices 13and 15. Although numerous types of voltage responsive devices areapplicable and appropriate, this particular embodiment is illustrated bya pair of cold cathode relay tubes 13 and 15 wherein each of the deviceshas a cathode 49 and 51, a starting anode 53 and 55. and an anode 57 and59.

The cathodes 49 and 51 of each of the voltage devices 13 and 15 aredirectly connected to each other and to circuit ground. The potentialacross the voltage divider 11 is applied through the current limitingresistors 45 and 4-7 to the starting anodes 53 and 55 of the voltagedevices 13 and 15. The anodes 57 and 59 are connected across the thirdwinding 27 of the transformer 9 by way of a pair of series circuitswhich include a current limiting resistor 61 and current responsivedevice 17 and a second current limiting resistor 63 and second currentresponsive device 19. Further, the current responsive device 17 includesa coil 65 shunted by a capacitor 67 and a normally open (N.O.) contact69 and, in a like manner, the device 19 includes a coil 71 shunted by acapacitor 73 and a normally open (N.O.) contact 75. The contacts 69 and75 are series connected in the control circuit to insure that bothcontacts are closed when the control circuit is operational.

As to the normal operation of the circuit 3, an alternating potentialapplied to the primary winding 21 of the transformer 9 causes apotential at the secondary winding 23 which activates the illuminationsource 31 of the lighting system 5. Simultaneously, the potential at thesecond winding 25 is applied to the regulating means 32 wherefrom asubstantially uniform potential is applied across the voltage divider11. In the voltage divider 11, the light responsive device 39 receivesthe reflected illumination from the source 31 of the lighting system 5and, under normal operating conditions, has a resistance substantiallyequal to the value of the fixed resistor 41. Thus, a substantially equalportion of the voltage across the divider 11 is applied to each of thestarting anodes 53 and 55 of the voltage responsive devices 13 and 15.

At the same time, a substantially equal portion of the potential acrossthe third winding 27 is applied to each of the anodes 57 and 59 of thevoltage devices 13 and 15 by way of the current responsive devices 17and 19 and current limiting resistors 61 and 63. Thus, current flowsthrough the current responsive devices 17 and 19 and causes the normallyopen contacts 69 and 75 to close thereby completing the circuitconnected to the apparatus to be controlled.

However, should the amount of illumination reflected onto the lightresponsive device 39 suddenly increase, the resistance value thereofwill decrease which will cause the voltage drop thereacross to decreaseand the potential on the voltage responsive device 13 to drop below theconduction sustaining potential. Thus, current flow through the currentresponsive device 17 will cease causing the contact 69 to open and theapparatus to become inoperative. In a somewhat similar manner, adecrease in the reflected illumination because of a defect in theillumination source 31 or for any one of a number of reasons, causes theresistance of the light responsive device 39 to increase and thepotential developed across the fixed resistor 41 to decrease. Thereupon,the voltage responsive device 15 ceases to operate which discontinuesthe current flow through the current responsive device 19 and causes thecontact 75 to open discontinuing the operation of the apparatus.

In one specific embodiment of the invention, the transformer 9 has aprimary winding 21 designed for the application of about 120 volts froman alternating potential source (not shown). Also, the transformer 9 hasa plurality of secondary windings including a first winding 23 providingabout 6.3 volts at 1.5 amperes thereacross, a second winding 25 havingabout 200 volts at 0.005 ampere, and a third winding 27 having about 250volts at 0.015 ampere.

The first winding 23 is bridged by an ordinary incandescent lamp such asa type GE1314 which acts as the illumination source 31 and, in thisinstance, is directed onto a relatively dark drum-like surface such asthe drum 77 of a duplicating machine.

The second winding 25 is bridged by a potential regulating means 32which includes a series conected resistor 33 in the order of about /2watt and ZZK-ohms and a pair of neon tubes 35 and 37. The tubes 35 and37 are of a type known as type NEZU and were selected because of theirready availability as well as their applicability to the othercomponents in the circuit 3. Since the desired voltage is about 200volts and such tubes have a firing voltage of about 100 volts and asustaining voltage of volts, a pair of tubes is required. Because anintrinsic characteristic of neon tubes is that the cathode material isphotosensitive, the emissive level thereof is greatly reduced in theabsence of illumination. Further, the firing or breakdown voltage ofsuch tubes rapidly approaches the sustaining voltage when the tubes areexposed to a relatively high level of illumination for an extendedperiod of time. Thus, it may be noted that the components of the circuit3, with the exception of the lighting system 5, are enclosed in alight-tight container 7 in this particular application.

It is because of this absence of illumination that the type NEZU tubeswere selected. It is well known that this tube type contains aradioactive additive which deters the deleterious effects of inadequateillumination and their inclusion within the light-tight container 7along with the previously mentioned cold cathode relay tubes 13 and 15provide a surrounding of sufficient stimulus for extended use.Additionally, should the stimulus provided by the relay tubes for theneon tubes 35 and 37 prove inadequate, a fourth winding 28 and a lamp 30may be included within the light-tight container 7 to compensate for theinsufficient illumination level.

The voltage divider 11 shunting the neon tubes 35 and 37 includes aphotoconductor 39, such as the readily available type 8143 and a fixedresistor 41 which is in the order of 75K-ohms and /2 watt. Thephotoconductor 39 forms a part of the lighting system 5 located externalto the light-tight container 7 and receives the reflected illuminationfrom the previously mentioned dark drum 77 whereon the light source 31is directed. This particular photoconductor has a resistance of about75K-ohms so long as the drum 77 remains in a normal darkened conditionand in conjunction with the resistor 41 equally divides the poten tialacross the divider 11.

The voltage responsive devices 13 and 15 are a pair of type 5823 coldcathode relay tubes, which were selected. because of their rapidresponse to a change in applied potential, and the current responsivedevices 17 and 19 are a pair of relays with normally open contacts 69and 75. These contacts 69 and 75 are series connected, and the coils 65and 71 thereof are shunted by a capacttors 67 and 73 to prevent chatteras is common in the art.

In the operation of the circuit 3, the alternating potential from thesource (not shown) may vary in the range of about to volts, and thispotential is maintained at a substantially constant value of about 200volts across the divider 11 by the regulating means 32. Thephotoconductor 39 and register 41 divide this potential which is equallyapplied to the cold cathode relay tubes 13 and 15, and, in conjunctionwith the potential applied thereto by the third winding 27, causescurrent to flow through the relays 17 and 19 and activate the controlcircuit.

However, if the amount of light reflected onto the photoconductivedevice 39 should suddenly increase because, for instance, of a piece ofwhite paper sticking to the normally darkened drum 77, the resistance ofthe photoconductor will decrease to a value of about 101(- ohms causingthe voltage applied to the relay tube 13 to drop and current conductiontherethrough to cease, the relay 17 also to cease functioning, thecontact 69 to open, and the operation of the apparatus controlledthereby to be interrupted. As previously mentioned, an increase inresistance of the photoconductor 39 operates in a similar manner toinactivate the relay tube 15, the relay 17, the contact 75, and theapparatus controlled thereby.

As to the fail-safe features of the circuit 3, it can be readily seenthat both of the normally open relays 17 and 19 must be activated inorder for the circuit 3 to function. Thus, a failure of the potentialsource or any of the windings 21, 23, 25, and 27 will interrupt theoperation. Further, a failure of the light source 31 will increase theresistance of the photoconductor 39 and discontinue the operation.

An increase or decrease in resitsance or failure of the components ofthe voltage divider 11 will inactivate the circuit 3 as will a failureof either of the voltage responsive devices 13 and 15. Also, a failureof either of the current responsive devices 17 and 19 because of poorcontacts 69 and 75 or failure of the coils 65 and 71 will inactivate theapparatus. Moreover, if the capacitors 67 and 73 should become opencircuited, which is highly un likely in electrolytic capacitors, therelays 17 and 19 will chatter while a short-circuited capacitor willinactivate the circuit 3.

Thus, there has been provided a control circuit having numerousadvantages and fail-safe features which are believed to be unobtainablein any similar device. This circuit assures the functioning of all ofthe components therein and provides a readily apparent dependabilitywhich is believed to be unavailable in any of the devices currentlyobtainable.

While there has been shown and described what is at present consideredthe preferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be made 4.A control circuit comprising: therein Without departing from theinvention as defined by a transformer having a plurality of secondarywindings the appended claims. including a first, second, and thirdwinding, said third What is claimed is:

winding having a center tap directly connected to 1. A control circuitcomprising: 5 circuit ground;

a transformer having a plurality of secondary windings an illuminationsource bridging said first winding, said including a first, second, andthird winding, said source being directed onto a reflective surface;third Winding having a center tap directly connected a pair of coldcathode relay tubes, each tube having an to circuit ground; anode, astarting anode, and a cathode, said cathodes an illumination sourcebridging said first winding; being directly connected to circuit ground;

a pair of voltage responsive devices, each device hava voltageregulating means including a series connected ing an anode electrode, astarting anode, and a cathresistor and a pair of neon tubes shuntingsaid secode electrode, said cathode electrodes being directly 0ndWinding, said regulating means and said pair of connected to circuitground; relay tubes being enclosed within a light-tight cona pair ofcurrent responsive devices, each device having tainer;

a normally open electrical contact and said contacts a pair of currentresponsive devices, each of said debeing series connected; and viceshaving a normally open electrical contact and circuit means connectingeach of said starting anodes said contacts being series connected; and

to said second winding and each of said anode eleccircuit meansconnecting each of said starting anodes trodes to said third winding byway of one of said to said voltage regulating means through aresiscurrent responsive devices, said means including a tor and each ofsaid anodes to said third winding voltage divider shunting said secondwinding and through a series connected resistor and one of said havingtherein an illumination responsive device current responsive devices,said circuit means inactivated by the reflected light from saidilluminacluding a voltage divider shunting at least a portion tionsource. of said voltage regulating means, said divider having 2. Acontrol circuit comprising: therein a series connected resistor and aphotocona transformer having a plurality of secondary windductor andsaid photoconductor being responsive to ings including a first, second,and third winding, said the reflected light of said illumination source.third winding having a center tap directly connected 5. A controlcircuit comprising: to circuit ground; a transformer having a pluralityof secondary windings an illumination source bridging said firstwinding; including a first, second, third, and fourth winding,

a pair of cold cathode relay tubes, each tube having an said thirdwinding having a center tap to circuit anode, a starting anode, and acathode, said cathodes ground; being directly connected to circuitground; an illumination source bridging said first Winding, said a pairof relays, each relay having a normally open source being directed ontoa light reflective surelectrical contact and a coil shunted by acapacitor, face; said contacts being series connected; and a pair ofcold cathode relay tubes, each tube having an circuit means connectingeach of said starting anodes anode, a starting anode, and a cathode,said cathodes to said second winding through a resistor and each beingdirectly connected to circuit ground; of said anodes to said thirdWinding thro gh a a voltage regulating means including -a seriesconse'ries connected resistor and a coil of one of said ted re istor anda pair of neon tubes shunting y Said means including 3 Voltage dividerShnntsaid second winding and a light source shunting said ing saidsecond winding, said divider including a fourth winding, said regulatingmeans and said pair series connected resistor and a photoconductor andof relay tubes being enclosed Within.a1ight-tight said photoconductorbeing responsive to the reflected mi r; light fI m aid i lumin i n o eapair of current responsive devices, each of said de- 3. A controlcircuit comprising: vices having a normally open electrical contact anda transformer having a plurality of secondary windings id onta t beinseries connected; and

including a first, Second, and third g, Said circuit means connectingeach of said starting anodes third Winding having a Center p directlyconnected to said voltage regulating means through a resistor to circuitground; and each of said anodes to said third Winding an illuminationS01E65 bridging Said first Winding, Said through a series connectedresistor and one of said Source being directed Onto a reflectiveSurface; current responsive devices, said circuit means ina P of VoltageTespOnsiVe devices, each device having eluding a voltage dividershunting at least a portion an anode electrode, a Starting anode, and aCathode of said voltage regulating means, said divider havelentfode,Said Cathode electrodes being directly C011- ing therein a seriesconnected resistor and a photonected to Circuit ground; conductor andsaid photoconductor being responsive a P of cllrrfint responsivedevicns, each of Said to the reflected light of said illuminationsource.

vices having a normally open electrical contact and said contacts beingseries connected; and References Cited circuit means including voltageregulating means UNITED STATES PATENTS shunqng saldi second wmdlngconnatlng g Donovan of said starting anodes to said voltage regulatinmeans through a resistor and each of said anodes 2,654,651 10/1953 Black340-285? to aid third winding through one of said current re- 2,687,4998/1954 Scothom 317-4 0 2,714,160 7/1955 MacDougall 317- X sponsivedevices, said means including a voltage 50 214 divider shunting at leasta portion of said regulating 3020413 2/1962 Hamelmk 2 means, saiddivider including a series connected re- 3,025,506 3/1962 McDonough317'- 0 sistor and a photoconductor and said photoconductor beingresponsive to the reflected light of said illumination source.

M. A. LEAVI'IT, Assistant Examiner.

5. A CONTROL CIRCUIT COMPRISING: A TRANSFORMER HAVING A PLURALITY OFSECONDARY WINDINGS INCLUDING A FIRST, SECOND, THIRD, AND FOURTH WINDING,SAID THIRD WINDING HAVING A CENTER TAP TO CIRCUIT GROUND; ANILLUMINATION SOURCE BRIDGING SAID FIRST WINDING, SAID SOURCE BEINGDIRECTED ONTO A LIGHT REFLECTIVE SURFACE; A PAIR OF COLD CATHODE RELAYTUBES, EACH TUBE HAVING AN ANODE, A STARTING ANODE, AND A CATHODE, SAIDCATHODES BEING DIRECTLY CONNECTED TO CIRCUIT GROUND; A VOLTAGEREGULATING MEANS INCLUDING A SERIES CONNECTED RESISTOR AND A PAIR OFNEON TUBES SHUNTING SAID SECOND WINDING AND A LIGHT SOURCE SHUNTING SAIDFOURTH WINDING, SAID REGULATING MEANS AND SAID PAIR OF RELAY TUBES BEINGENCLOSED WITHIN A LIGHT-TIGHT CONTAINER; A PAIR OF CURRENT RESPONSIVEDEVICES, EACH OF SAID DEVICES HAVING A NORMALLY OPEN ELECTRICAL CONTACTAND SAID CONTACTS BEING SERIES CONNECTED; AND CIRCUIT MEANS CONNECTINGEACH OF SAID STARTING ANODES TO SAID VOLTAGE REGULATING MEANS THROUGH ARESISTOR AND EACH OF SAID ANODES TO SAID THIRD WINDING THROUGH A SERIESCONNECTED RESISTOR AND ONE OF SAID CURRENT RESPONSIVE, DEVICES, SAIDCIRCUIT MEANS INCLUDING A VOLTAGE DIVIDER SHUNTING AT LEAST A PORTION OFSAID VOLTAGE REGULATING MEANS, SAID DIVIDER HAVING THEREIN A SERIESCONNECTED RESISTOR AND A PHOTOCONDUCTOR AND SAID PHOTOCONDUCTOR BEINGRESPONSIVE TO THE REFLECTED LIGHT OF SAID ILLUMINATION SOURCE.